New Zealand’s steep mountain streams and forested rivers are far too swift and rough for most ducks. However the whio (Hymenolaimus malacorhynchos) takes churning rapids, huge boulders and fallen logs in its stride. It has specialised features that are well adapted to this habitat.
Blue-grey with a flecked chestnut breast, the whio blends into its rock and water environment, often hiding in overhanging vegetation on the banks where it roosts and nests.
The whio has large, webbed feet to give it power in fast-flowing water, and well-developed claws for rough terrain. Even hatchlings have oversize feet and strong legs, ready to swim in swift currents and jump onto large rocks and logs.
The duck eats insect larvae – including caddisfly, mayfly and stonefly larvae – which it scrapes off rocks underwater. Its bony bill is protected from abrasion by a fleshy flap.
Whio are about 53 centimetres long. Males weigh 900 grams and females 750 grams.
A whio pair’s territory is typically more than a kilometre of waterway, which the male defends with a high territorial whistle that carries beyond noisy rapids – hence its Māori name, which means whistle. The female has a grating alarm call.
Whio often stay under cover in daylight, feeding at dawn and dusk. They occasionally fly low along their stretch of water by day. Juveniles seeking a new territory may make longer flights at night to a neighbouring valley or over a mountain pass.
Driving to Pūkaha Mt Bruce National Wildlife Centre with a brood of two-week-old whio, wildlife officer Tom Steel was astounded when the strong-legged little ducklings leapt over from the rear seat and climbed onto his shoulder. Next thing they scrambled down and crawled under the clutch pedal.
Whio nest under cover on steep stream banks. Their ducklings are not brooded, but they reduce heat loss by huddling together. Adults guard them attentively as they learn to swim and feed in swift water.
Māori did not find whio tasty, and the birds were still widespread and abundant in the early days of European settlement. But they lost ground as lowland forest was progressively cleared. Whio were still quite common in the 1950s, and their ongoing decline is mainly due to stoats. Introduced trout probably compete for food.
Whio are a protected endemic species. They are classified as nationally endangered, and the total population was estimated at 2,500 in 2004.
The Department of Conservation is working to stop their decline. Stoat control at several sites is allowing more ducklings to survive, and some have been raised in captivity to boost numbers in the wild. A population has also been re-established on Mt Taranaki (Mt Egmont).